replace function error fix (#87)

Author: gurinder39

public/app/features/templating/template_srv.ts

@@ -331,11 +331,13 @@ export class TemplateSrv implements BaseTemplateSrv {
   private _replaceWithVariableRegex(text: string, format: string | Function | undefined, replace: ReplaceFunction) {
     this.regex.lastIndex = 0;
 
-    return text.replace(this.regex, (match, var1, var2, fmt2, var3, fieldPath, fmt3) => {
-      const variableName = var1 || var2 || var3;
-      const fmt = fmt2 || fmt3 || format;
-      return replace(match, variableName, fieldPath, fmt);
-    });
+    return (
+      text?.replace(this.regex, (match, var1, var2, fmt2, var3, fieldPath, fmt3) => {
+        const variableName = var1 || var2 || var3;
+        const fmt = fmt2 || fmt3 || format;
+        return replace(match, variableName, fieldPath, fmt);
+      }) || ''
+    );
   }
 
   isAllValue(value: any) {